The present invention generally relates to a method and apparatus for recording signals such as currents and voltages, positions of circuit breakers or other switching mechanisms, and output signals of protective relays and other protection and control devices for archiving and analysis purposes in electrical power systems. More particularly, the present invention allows recording both physical signals and other signals represented in digital data packets and shared between a number of devices that either send or receive such information over a digital communication means.
Electrical power distribution systems are monitored by protective and control devices. Protective relays are well-known for monitoring and providing protective control of electrical power distribution systems. As used herein, the term “electrical power distribution system” or “electrical power system” includes systems, and components of systems, for the generation, transmission, and/or distribution of electrical power to customers. Protective and control device primarily maintain operability of the electrical power system and ensure reliable delivery of electrical power to the customers. When the protective and control devices are required to operate, they must respond automatically within short periods of time.
Protective relays, for example, are installed to isolate a faulty component of the electrical power system in the event of equipment failure to maintain the remaining system in tact and to prevent the damage from spreading any further. Such relays are typically connected at a point on the electrical distribution system, to monitor current, voltage, or other parameters, and to provide protective control (e.g., by providing a control signal to cause power to be removed from the portion of the system with which the relay is associated) in the event that certain specified conditions occur. The protective relay isolates the faulty component by opening at least one associated circuit breaker. Thus, protective relays are in operable communication with power system currents and voltages, as well as certain auxiliary signals via instrument transformers and other sensors to detect such faulty components.
Equipment in an electrical power system can occasionally fail, as discussed above. Furthermore, the protective and control devices and instrument transformers installed to detect such failures may fail themselves, which may result in an unnecessary operation of the protection and control devices or a failure of the protective and control devices to operate when their operation is required. In such a complex environment, it is necessary to monitor both the primary power system equipment and associated secondary protection and control devices using independent recording devices (see FIG. 1). These include fault recorders, which primarily include digital fault recorders (DFRs) and sequence of events (SOE or SER) recorders. The DFRs record analog signals including currents and voltages predominantly, but may include other electrical or electrically represented quantities as well. The SOE recorders record changes in status of on/off signals including breaker position, operation of protective relays, trip signals for circuit breakers, but not limited thereto.
The operation of recording is accomplished by storing the relevant information on a permanent storage medium, such as a non-volatile memory in the case of digital devices, accompanied with storing reasonably accurate timing information. The timing and synchronization aspects are important as they allow analysis of the records produced at different sites in the electrical power systems.
In the past few years digital protection and control devices, or even instrument transformers, have emerged to work with power system signals that are not in the form of physical or analog quantities, but in the form of digital data packets exchanged over appropriate communication channels. Such communication channels include, but are not limited to, direct fiber optic connections, Ethernet, and serial ports, for example. Not only is the new form of representation different in nature (e.g., digital versus analog), the communicating protection and control devices use the digitally exchanged signals differently. For example, it is quite common to re-send the same information several times in order to make sure the information arrives at the intended destination; channel integrity messages may be exchanged to monitor the communication means; and check-sums or other security means may be attached to the body of the message to ensure integrity of the data. All of this creates new challenges for the task of fault recording and sequence of event recording.
As a rule, protection and control devices are capable of some level of recording digitally exchanged signals. However, the primary function of the protection and control devices is to, first, provide protection and/or control. As such, the protection and control devices themselves are subject to monitoring and their own records should not be fully trusted. Second, the protection and control devices support only those types of physical connections and communication connections that are required to perform their primary functions, not generic recording functions. Third, the data storage capabilities of the protection and control devices are typically limited allowing short-term recording only.
Accordingly, there is a need for a recording device and method that supports a variety of digital communication means and primarily functions to record faults and sequence of events in an electrical power system.